Field of the Invention
This invention is generally directed to electrical resistance heaters of the type which are designed to be placed against a surface to be heated and which include heating elements which are embedded in overlapping relationship within a pliable material. More specifically, however, the present invention is directed to a surface contact electrical heater having a pair of heating elements each connected to a common source of power supply, and wherein a first heating element is energized in response to a separate thermostat connected thereto and wherein a second heating element is continuously activated whenever the heater is connected to the source of power supply. Thus, the heater operates at a lower wattage density when only the second or continuously activated heating element is energized and at a higher wattage density when the thermostat causes the first heating element to be energized.
The dual resistance heaters of the present invention are uniquely designed for use as engine oil pan heaters, however, they may also be used in many other areas including, but not limited to, transmission fluid heaters, battery heaters, livestock trough heaters, fluid pipe and conduit heaters and the like.
History of the Related Art
Electrical resistance heaters having multiple heating elements have long been known. Many such devices incorporate two or more separate resistance elements which are connected to a common source of power. In some instances, the resistance elements are designed to expand the effective heat exchange area of the heater. Examples of such heaters are disclosed in U.S. Pat. No. 4,279,255 to Hoffman and U.S. Pat. No. 4,633,061 to Arikawa. Generally, such heaters are designed to only increase the area of heat exchange and not the heat exchange density of the overall surface contact area of the heater. In the Hoffman patent, provision is made for even reducing the power density and thus the heat exchange rate of the heating elements as the size of the heat exchange area is increased by operation or energization of supplemental resistance heating elements.
Other types of electrical resistance heaters having overlapping heating elements have also been developed. Such heaters are generally designed to increase the heat exchange rate of the heater either over a portion of the surface area or along the entire surface area of the heater. U.S. Pat. No. 3,739,142 to Johns discloses an electrical heating blanket having a supplemental heating element overlapping the primary heating element so as to provide additional heat along a specific portion of the blanket, in this instance, along the area of the blanket that would be used to cover an individual's feet. The patent discloses that each of the heating elements is provided with a separate thermostatic control thus increasing the overall cost of the unit.
Other U.S. Pat. No. including 2,619,580 to Pontiere, U.S. Pat. No. 3,634,655 to Jordan and U.S. Pat. No. 4,788,417 to Graflind disclose generally flexible resistance heaters having arranged in overlapping relationship along substantially the entire area of the heater, with the exception of the patent to Graflind wherein only two overlapping elements of a multiplicity of such elements are in overlapping relationship at any point along the surface area of the heater. In each of the references either separate controls are provided for each of the heating elements or a single thermostat provided for controlling each of the heating elements. Further, in each case, the heating elements are generally designed to supply the same amount of heat output per unit of heat exchange surface area.
The aforementioned flexible resistance heaters are primarily designed for use as wraps or blankets, floor coverings and the like, and are limited in their field of use. For example, such heaters could not be used in the automotive field for use as oil pan or transmission fluid heaters. In many areas where the climate during the winter is severe with temperatures frequently being below freezing, it is necessary to provide supplemental heat sources for maintaining the oil within an internal combustion engine at a temperature which will allow the engine to be started easily. Numerous oil pan heaters have been designed and such heaters generally utilize a single heating element which operates at a given wattage for transferring heat to the oil through the oil pan by conduction. The heaters are attached or mounted to the engines in a variety of ways and, in some instances, are magnetically or adhesively secured.
U.S. Pat. No. 1,764,021 to Jackson discloses an oil heater which was designed to clamp below the oil pan and included a heating coil which heated the oil through both radiation and convection. The heater was operable at different energy settings to regulate the rate at which the oil was heated. U.S. Pat. No. 2,698,374 to Carpenter is another heating device designed to be attached to the side of an engine oil pan. The Carpenter heater incorporated a thermostat designed to monitor the temperature of the oil through the wall of the oil pan and was operable to deenergize a heating element once the oil reached a prescribed temperature.
One of the more frequent problems encountered with conventional engine oil heaters of the type which are mounted exteriorly of an oil pan is that the surface contact area between the heater and the oil pan is frequently inefficient thereby requiring wasted energy to be consumed in an effort to heat the oil to a desired temperature. Often, poor contact between the heating elements and the oil pan required that the heat exchange be accomplished more by radiation than by direct conduction.
Another problem encountered with many conventional exterior oil pan heaters is that they are designed to be regulated totally by thermostatic control so that energy is applied whenever the heater is in operation until such time as the heat within the oil pan reaches a predetermined temperature being regulated by the thermostat. In many instances it is not necessary that the oil be heated to the degree to which the thermostat has been set and thus, in some instances, either energy is wasted while, in others, potential overheating of the engine oil is possible.
In order to improve the surface contact of resistance heaters relative to the side walls of an oil pan or an engine block to thereby increase the effective conductive heat transfer, flexible reinforced silicon heaters were developed having foil heating elements chemically etched therein. Such heaters not only allowed flexibility so that the heaters could be secured following the natural contour of the oil pan or engine component, but also allowed for the heaters to be adhesively secured to the oil pan or other engine component thus decreasing the conductive heat losses. Energy to the heating elements of such devices are conventionally regulated by a thermostatic control switch. In the U.S. Pat. No. 5,017,758 to Kirkman et al., another type of flexible resistance oil pan heater is disclosed which was designed to prevent excessive temperatures from breaking down the oil within an automotive engine. This patent also utilized an etched or foil type heating element sandwiched between reinforced silicone layers, however, the heater specifically excluded the use of any thermostatic control. The heater is designed to retain the oil temperature between 170.degree. F. and 300.degree. F. utilizing a heating element having a uniform power density of at least 15 watts per square inch and a constant power output of about 20 to 40 watts per quart of oil within the vehicle oil pan or reservoir. Unfortunately, utilizing such a heating element requires a constant power output that it is not always necessary to maintain temperatures within the automotive engine at sufficient levels to allow for engine start-up in cold climate.